Ion guides are known which are used to transport ions between different regions in a mass spectrometer. For example, an ion guide may be used to transport ions from or to an ion source, collision cell, mass analyser or between regions having different gas pressures. Ion guides may also be used as gas cells to collisionally cool or heat continuous beams or packets of ions by colliding the ions with a gas. Collisional cooling reduces the average kinetic energy of the ions which is advantageous, for example, for subsequent mass analysis of the ions using a Time of Flight (“TOF”) mass analyser. Alternatively, ions may be collisionally heated within an ion guide during transportation between two regions so as to cause the ions to fragment. The product, daughter or fragment ions may be mass analysed in order to determine the chemical structure of the associated parent ions.
Conventional ion guides may comprise a multipole parallel rod set of electrodes e.g. a quadrupole, hexapole or higher order rod set or a stacked concentric circular ring set of electrodes (i.e. an “ion tunnel” ion guide) comprising a plurality of electrodes having apertures through which ions are transmitted in use. AC or RF voltages are applied to opposing rods in a multipole rod set or to alternate rings in an ion tunnel ion guide such that the voltages applied to the opposing rods or alternate rings have opposite phases. The geometries of the electrodes in a multipole rod set or a ring set ion guide are arranged so that inhomogeneous AC/RF electric fields generate pseudo-potential wells or channels within the ion guide. The ions are preferably confined in these potential wells and are guided through the ion guide.
A significant issue with multipole rod set ion guides such as quadrupole, hexapole or octopole rod sets is that they are relatively complex arrangements and hence are comparatively expensive to manufacture. The complexity and expense becomes a particularly significant problem if the multipole rod set ion guide is intended to transport ions over a relatively long distance.
Another known form of ion guide is an Electrostatic Particle Guide (“EPG”) which comprises a cylindrical electrode having a guide wire running along the central axis of the cylinder. Different static DC voltages may be applied to the guide wire and the conductive outer cylindrical electrode so that, for example, the guide wire may be connected to a DC potential which attracts ions and the outer cylindrical electrode may be connected to a DC potential which repels ions. Injected ions will follow elliptical paths around the guide wire under conditions of high vacuum otherwise the velocity of the ions would be dampened by collisions with gas molecules and the ions would discharge upon hitting the guide wire. The potential difference between the guide wire and the outer cylindrical electrode generates a steep logarithmic potential well within the ion guide with the centre of the potential well being located at the guide wire. The guide wire may, for positively charged ions, be at a lower potential than the outer cylindrical electrode so that positive ions are attracted radially inwards towards the guide wire electrode. Negatively charged ions within the electrostatic particle guide will be attracted towards the outer cylindrical electrode and will be lost. Alternatively, the guide wire may be maintained at a higher potential relative to the outer cylindrical electrode so that negative ions are attracted radially inwards towards the guide wire and positively charged ions are repelled.
Some of the positive or negative ions which are attracted to the guide wire enter into stable orbits about the guide wire along the length of the ion guide. However, other ions will strike the guide wire and will be lost. The transmission losses due to ion collisions with the guide wire will depend upon the radius of the guide wire and the energy and spatial distribution of ions entering the guide wire ion guide. Significant transmission losses will occur when ions have kinetic energies in the radial direction which are greater than the depth of the potential well within the cylindrical electrode. These energetic ions will tend to strike the inner surface of the cylindrical electrode and will become neutralised and lost. Further significant transmission losses are also observed if the conventional guide wire ion guide is operated at relatively high pressures. At higher pressures the mean free path between collisions between ions and neutral gas molecules is significantly shorter than the length of the guide wire ion guide and hence the ions will tend to collide with the gas molecules many times before leaving the ion guide. These collisions cause the ions to lose kinetic energy which results in the ions spiraling into the guide wire and thus being lost.
In view of the above mentioned problems, known guide wire ion guides are only used to transport ions through regions of relatively low gas pressure wherein collisions between ions and gas molecules are unlikely.
It is therefore desired to provide an improved guide wire ion guide and in particular a guide wire ion guide which is suitable for use at relatively high pressures.